1. Field of the Invention
The present invention relates to an apparatus for supplying raw material used in single crystal growth by means of a CZ method, and more particularly, to an apparatus for supplying raw material used in charging granular/lump polycrystalline raw material into a crucible.
2. Description of the Background Art
A silicon single crystal, which serves as a material of a semiconductor device, is exclusively used by means of a CZ method for the industrial purpose. In single crystal growth by means of a CZ method, solid poly-silicon initially charged in a crucible is molten by heating with a heater. When a raw material melt is formed in such a way in the crucible, a seed crystal is dipped into the raw material melt in the crucible and starting in this state, the seed is raised while rotating the seed and the crucible, thereby growing a silicon single crystal in the shape of a cylinder at and below the lower end of the seed. As a solid raw material initially charged in the crucible, there are used cut rods, lumps, granules and others of the poly-silicon, either alone or in combination.
In such single crystal growth by means of a CZ method, multipulling-up has been conceived for the purpose to reduce a crucible cost and others. This is a technique in which after a single crystal is pulled up, a solid raw material is additionally charged into a remaining raw material melt in the crucible to thereby again form a raw material melt at the same amount as when the solid raw material is initially charged in the crucible, which is followed by repetition of pulling-up of a single crystal with the result of an increased number of pulling-up times per one crucible. In this technique, additional charge of a solid raw material is called recharge. Techniques to recharge cut rods are described in Patent JP B 94(H6)-31193 and JP A 96(H8)-310892, and techniques to recharge a granular raw material are described in JP A 87(S62)-260791, JP A 96(H8)-143392 and JP A 97(H9)-227288.
Since not only are cut rods higher in unit price by weight as compared with that of a granular raw material, but mechanical processing at an end portion of a cut rod is also necessary in order to hang down the cut rod above a crucible, leading to a considerably high cost in total. Therefore, it is recommended to use a granular raw material from the viewpoint of cost effectiveness. A granular raw material has, however, a larger surface area. A problem has also arisen that an oxide film formed on a larger surface area of the granules is burst in a raw material melt when a raw material is charged and not only is splashed melt attached onto constituent members to be a solid matter, but the attached solid matter falls off onto the surface of the melt to cause crystal defects such as dislocations in a growing crystal.
Judging from the circumstances described above collectively, a lump material, which is larger in size of each piece than a granular raw material, can be expected as a lower cost recharge raw material as replacement for cut rods. Since not only is a lump raw material as low in cost as a granular raw material, but the surface area of a lump raw material less than a granular raw material, no problem of the splashing occurs. A lump raw material, however, does not necessarily have any problem. In a case of a lump raw material, the following problem has arisen with respect to a raw material supply apparatus.
A necessity arises for a granular raw material to be guided into a crucible using a charging tube when the granular raw material is additionally charged into a remaining melt in the crucible. As charging tubes, there are available an inclined charging tube installed obliquely downward from above a crucible and a vertical charging tube installed vertically from right above the crucible. In a most recent pulling furnace, arrangement of the inclined charging tube causes a problem of interference with many of accessory members such as a heat shielding member above the crucible because of the presence thereof. Hence, additional charge of the granular raw material can be said to be desirably conducted with the vertical charging tube as the latter choice in the central section of the crucible.
In additional charging of a lump raw material using the vertical charging tube, melt splashing caused by the raw material is problematical, however. This is because a lump raw material is heavier than a granular raw material and, in a case of the vertical charging tube, a falling speed is larger in the vertical charging tube than in the inclined charging tube, so kinetic energy is increased when being charged.
As a measure for decreasing kinetic energy, when a lump raw material is charged, down to a level at which melt splashing is not problematical, it is conceived to bend the vertical charging tube at a site thereof part way downward. If the vertical charging tube is bent between both ends thereof, a falling raw material strikes against the bent portion to release part of kinetic energy to thereby decrease energy when being charged. A vertical charging tube is, however, made from a quartz pipe from the viewpoint of contamination. Moreover, a quartz pipe is low in mechanical strength; therefore, wear loss or breakage thereof occurs in the bent portion against which a falling raw material strikes.